[NTG-context] URLs break bib export

[Rik Kabel]

The new tagged biblography export is very nice, but there is a routine 
(marked UGLY in the source) that causes processing to fail with 
otherwise-valid input. The failure message is

...eXt/tex/texmf-context/tex/context/base/mkiv/back-exp.lua:3889: invalid capture index
stack traceback:
         [C]: in function 'gsub'
         ...eXt/tex/texmf-context/tex/context/base/mkiv/back-exp.lua:3889: in function <...eXt/tex/texmf-context/tex/context/base/mkiv/back-exp.lua:3747>
         (...tail calls...)

and two URLs which cause this problem are:

    url         = {http://www.jerwood-no.org.uk/pdf/Dunning%20Kruger.pdf},
    url         = {http://oll.libertyfund.org/?option=com_staticxt&staticfile=show.php\%3Fcollection=46&Itemid=27},

These URLs caused no problem with PDF output  or in the previous 
exports, although they were not printed and exist only in the bibliography.

Do I need to protect the special characters in the bib file, or is this 
a problem with the UGLY code?

-- 
Rik